Definition
Encryption is the application of a cryptographic algorithm to transform plaintext into ciphertext, rendering the data unreadable without the appropriate key. It is the primary mechanism by which VPNs deliver confidentiality. The algorithm is the cipher; the parameter is the key; together they define a one-way door that opens only when presented with the right key.
Encryption in a VPN
Every byte that enters a VPN tunnel is encrypted before it leaves your device. The cipher is symmetric (AES, ChaCha20) for speed; the key was negotiated through an asymmetric handshake (Diffie-Hellman) at the start of the session. The receiving end — the VPN server — decrypts the packet, reads the original destination, and forwards it onto the public internet. Replies follow the reverse path, encrypted on the server and decrypted at the client.
What Encryption Does Not Do
Encryption protects data in transit, between the encryption point and the decryption point. It does not hide that the encrypted communication is happening, it does not protect data once it leaves the tunnel onto the open internet, and it does not prevent the endpoints — your device and the destination server — from reading the plaintext themselves. Encryption is a single layer in defence-in-depth, not a complete privacy solution.